Measured and predicted electron cyclotron power deposition and current drive widths in DIII-D

Abstract

Experiments in DIII-D have measured the width of the power deposition and current drive profiles for electron cyclotron (EC) waves by modulating the gyrotrons and fitting the periodic response of the electron temperature profile or motional Stark effect signals to a linearized diffusion equation. The measured EC power deposition widths are scattered around the predicted values, including both narrower and wider cases. The average width for single gyrotron modulation is ≈88% of the theoretical value from ray tracing, while the average width for multiple gyrotron modulation is ≈127% of theory; aiming misalignment between the different EC launchers may explain why multi-gyrotron measurements are wider than for single gyrotrons. The measured EC current drive widths have nearly the same amount of profile broadening (≈131% of theory, on average) as does multi-gyrotron power deposition. Finally, while the measured locations of EC current drive are well centered on the expected values, the measured heating locations are skewed to larger radius. This difference may be due to rapid heat pulse propagation causing an inherent outward bias to the deduced power deposition profile, whereas magnetic flux diffusion has less effect on current drive measurements.